Of course the only way to be sure would be a comparison with the lossless version, but to me it doesn't seem a compression artefact. It seems more an imperfection in the recording, or some kind of "mechanical" noise from the bow strokes, maybe the microphones were placed a little too near.

In my experience, trying to spot artefacts to ABX on AAC @256Kbps has instead leaded me to find a lot of similar imperfections in the original recordings!